L-Tartaric acid was produced by transformation of cis-epoxysuccinic acid using Nocardia tartaricans ATCC 31191 in a molar yield of 100%. cis-Epoxysuccinic acid was prepared by epoxidation of maleic acid in the presence of molybdate or tungstate ions. The effect of these ions on the subsequent activity of the cis-epoxysuccinate hydrolase was determined in both intact cells and cell-free extracts of Nocardia tartaricans. Based on kinetic studies, the tungstate ions behaved as a strong inhibitor while the molybdate ions as a stimulator of the enzyme.